Neutrino and Cosmology

Neutrinos are, with the photons, the most abundant particles in the Universe. In the big bang theory (the “standard” model of the Universe), light neutrinos have thermally decoupled from the other forms of matter (quarks and leptons) approximately 1 second after the big bang, when the temperature decreased to about 1010 K (~ MeV). They constitute the cosmic neutrino background, the first witnesses still alive of the big bang (the photon background, the so-called cosmic microwave background (CMB), is “younger” since the decoupling took place 380 000 years later). Their temperature has now decreased to 1.95 K and their density (all the species together) is presently 330 neutrinos per cm3.

The energy of these relic neutrinos is so low (mean energy of ~0.1 meV) that their cross section is very small (order of 10-60 cm2) and it has not yet been possible to detect them. Some hope to detect relic neutrinos emerged when it was realized that coherent interaction can enhance the cross section. Indeed these low energy neutrinos have a macroscopic de Broglie wave length (≈ 0.5 mm). Several ingeneous ideas has been proposed since more than 50 years (see the review by Gelmini in 2004 [Gel04]) :

– mechanical momentun transfer on macroscopic object [Oph74, Lew80], but this simple idea was shown to be highly reduced at the level of GF2 [Cab82]

Steven Weinberg proposed in 1962 to look for the signal for relic neutrino capture on tritium [Wei62]. Raghavan revised this idea [Rag07] and today the Ptolemy collaboration [Bet13] is making the first steps toward its experimental realization.

It is also worth noticing that the interaction of (still undiscovered) ultra-high energy neutrinos (~ 1021 eV) with the relic neutrinos to produce a Z0, signed by a huge increase in the cross section [Wei82] (see [Rin01] for a recent review).

If no direct method has proved the existence of the relic neutrinos, there are indirect approaches which give confidence that they really exist:

the big bang nucleosynthesis, which generates the light nuclei (D, 3He, 4He, 7Li) [Wag67], asks for a number of neutrino species smaller than 5 [Ste77].

the cosmic neutrino background affects the evolution of CMB anisotropies and the structure formation in the Universe. The last results of the Planck satellite also require a number of neutrinos close to 3 [Ade13]; in addition, Planck and other cosmological experiments give a very constraining upper limit on the mass of the neutrinos.

Neutrinos as dark matter candidates ?

Since the works of Zwicky in 1933 and Rubin in 1970, we know that some “dark matter”, observed only through its gravitational interaction, is present in the universe and is about 6 times more abundant than the visible matter. As soon as the solar neutrino deficit was supposed to be the consequence of neutrino oscillation and that neutrinos are thus massive, physicists, knowing that there are many neutrinos (about 330 per cm3) of cosmological origin, proposed that neutrinos could be the main component of the dark matter. But, it required the neutrino mass to be at least of 10 eV. The lastest results on the neutrino oscillation parameters and the strong cosmological constraints on the neutrino mass provided by experiments like Planck have excluded this hypothesis and the hope to have found a good candidate for the dark matter.

The idea that a fourth neutrino which would be massive (~keV) and sterile has been popular in the recent years. We need more experimental proofs before considering that they could constitute part of dark matter.